讲解一下这个题目---**Extracted Content:** **Question ID:** 7. (2023·全国甲卷·25) **Question Stem and Problem Description:** As shown in the figure, a smooth U-shaped metal rail is fixed on a horizontal table. The spacing between its parallel parts is $l$. The outermost right end of the rail is aligned with the right edge of the table. The resistance of the rail is negligible. The region where the rail is located has a uniform magnetic field directed vertically upwards, with a magnetic induction strength of $B$. A metal rod P, with mass $m$, resistance $R$, and length $l$, is at rest on the rail. An insulating rod Q, with mass $3m$, is located to the left of P on the rail. It moves towards P with a speed of $v_0$ and undergoes an elastic collision with P. The collision time is very short. After the collision, P and Q slide out from the outermost right end of the rail one after another, and land at the same point on the ground. While P is moving on the rail, its two ends maintain good contact with the rail, and P and Q remain parallel at all times. Air resistance is negligible. **Question (Part 1):** (1) 求: 金属棒P滑出导轨时的速度大小 (Calculate: The magnitude of the velocity of metal rod P when it slides out of the rail) **Chart/Diagram Description:** * **Type:** A schematic diagram depicting a physics experimental setup in a 3D isometric view. * **Main Elements:** * **Horizontal Table/Surface:** A flat, rectangular surface representing a table, on which the setup is placed. * **U-shaped Metal Rail:** A U-shaped track made of metal, fixed on the table. The parallel arms of the U-shape extend towards the right. * **Metal Rod P:** A straight rod labeled "P", positioned horizontally on the parallel parts of the U-shaped rail, closer to the right (open) end of the rail. * **Insulating Rod Q:** Another straight rod labeled "Q", positioned horizontally on the parallel parts of the U-shaped rail, to the left of rod P. * **Magnetic Field (B):** Several arrows are drawn vertically upwards from the plane of the rail, indicating a magnetic field. One of these arrows is labeled "B". * **Relative Position and Direction:** * The U-shaped rail is placed flat on the horizontal table. * Rod Q is situated to the left of rod P, with both rods resting on the parallel segments of the rail. * The magnetic field is uniform and points perpendicularly upwards from the surface where the rail and rods are located. * **Labels and Annotations:** P, Q, B.